U.S. patent number 5,834,864 [Application Number 08/527,479] was granted by the patent office on 1998-11-10 for magnetic micro-mover.
This patent grant is currently assigned to Hewlett Packard Company. Invention is credited to Jobst Brandt, Victor W. Hesterman, Robert G. Walmsley.
United States Patent |
5,834,864 |
Hesterman , et al. |
November 10, 1998 |
Magnetic micro-mover
Abstract
A specially designed, forceful, compact magnetic micro-mover
includes at least one etched single crystal silicon plate having
integral springs and stable, low internal stress. A structure of
springs which support a rectangular plate are etched in silicon.
The plate is driven by planar electromagnetic actuation. The
micro-mover consists of a specific etched silicon spring in
combination with a planar electromagnetic moving coil actuator that
is capable of generating forces greater than 50 mN, while
dissipating less than 1 W peak power. The micro-mover also includes
a plate suspension system that facilitates the required plate
motions and resolution, while preventing changes in plate spacing
from effects such as external acceleration, temperature, humidity,
and aging; and includes an actuator that appropriately moves the
plates in response to electrical signals, such that there is only
minimal cross-talk between axes of motion.
Inventors: |
Hesterman; Victor W. (Los Alto
Hills, CA), Walmsley; Robert G. (Palo Alto, CA), Brandt;
Jobst (Palo Alto, CA) |
Assignee: |
Hewlett Packard Company (Palo
Alto, CA)
|
Family
ID: |
24101624 |
Appl.
No.: |
08/527,479 |
Filed: |
September 13, 1995 |
Current U.S.
Class: |
310/40MM;
360/78.05; 360/78.12; G9B/21.012; G9B/21.003; G9B/21;
G9B/19.027 |
Current CPC
Class: |
G01Q
10/04 (20130101); G11B 21/00 (20130101); H02K
99/00 (20161101); B82Y 35/00 (20130101); G11B
21/02 (20130101); G11B 21/08 (20130101); G02B
21/32 (20130101); H02K 33/18 (20130101); G11B
19/20 (20130101); G02B 26/085 (20130101); H02K
2201/18 (20130101); G11B 9/1418 (20130101); H02K
41/035 (20130101); H02K 11/00 (20130101); B82Y
10/00 (20130101); G11B 9/1436 (20130101) |
Current International
Class: |
G11B
21/08 (20060101); G11B 21/00 (20060101); G11B
19/20 (20060101); G02B 21/32 (20060101); G02B
26/08 (20060101); G11B 21/02 (20060101); H02K
33/18 (20060101); H02K 57/00 (20060101); H02K
41/035 (20060101); H02K 11/00 (20060101); G11B
9/00 (20060101); G11B 005/596 () |
Field of
Search: |
;310/12,4MM
;360/78.05,109,105,78.12
;156/655.1,656.1,657.1,659.11,661.1,662.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0371661A2 |
|
Nov 1989 |
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EP |
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0517061A1 |
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May 1992 |
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EP |
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0578228A3 |
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Jul 1993 |
|
EP |
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0686863A1 |
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Aug 1994 |
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EP |
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19601018A1 |
|
Jan 1995 |
|
DE |
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WO 92/20842 |
|
Nov 1992 |
|
WO |
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WO 96/04525 |
|
Feb 1996 |
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WO |
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Other References
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R. Hantke, Chemnitz, "Elektrostatischer Mikroaktor", 449 F&M
Feinwerktechnik & Messtechnik 101 (1993) Mai, No.5, Munchen,
DE. .
W. Dotzel, R. Kiehnscherf, Chemnitz, und T. Ziegler, Berlin,
"Mikromechanische Aktoren magnetisch betatigen", 449 F&M
Feinwerktecnik & Messtechnik 100 (1992) Nov., No. 11, Munchen,
DE. .
Wagner et al, "Permanent Magnet Micromotors on Silicon Substrates,"
Journal of Microelectromechanical Systems, vol. 2, No. 1, Mar.
1993, pp. 23-29. .
Ahn et al., "A Planar Variable Reluctance Magnetic Micromotor with
Fully Integrated Stator and Coils," Journal of
Microelectromechanical Systems, vol. 3, No. 4, Dec. 1993, pp.
165-173. .
Busch-Vishniac, "The case for magnetically driven microactuators,"
Sensors and Actuators, A (1992) pp. 207-220 month unknown. .
Fujita et al., "New Opportunities for Micro Actuators," IEEE
(1991), pp. 14-20 month unknown. .
Fan et al., "Batch-Fabricated Milli-Actuators," IEEE (1993) pp.
179-183 month unknown. .
Brennan et al., "Large Displacement Linear Actuator," IEEE (1990)
pp. 135-139 month unknown. .
Takeshima et al., "Electrostatic Parallelogram Actuators," IEEE
(1991) pp. 63-66 month unknown. .
Hirano et al., "Design, Fabrication, and Operation of Submicron Gap
Comb-Drive Microactuators," Journal of Microelectromechanical
Systems, vol. 1, No. 1, Mar. 1992, pp. 52-59. .
Niino et al., "High-Power and High-Efficiency Electrostatic
Actuator," IEEE (1993) pp. 236-241 month unknown. .
Minami et al., "Fabrication of Distributed Electrostatic Micro
Actuator (DEMA)," Journal of Microelectromechanical Systems, vol.
2, No. 3, Sep. 1993, pp. 121-127. .
Wallrabe et al., "Theoretical and Experimental Results of an
Electrosttic Micro Motor with Large Gear Ratio Fabricated by the
Liga Process," Micro Electro Mechanical Systems '92, Travemunde
(Germany), Feb. 4-7, 1992, pp. 139-140. .
Wise, "Integrated Silicon Sensors: Technology and Microstructures,"
IEEE 1991, pp. 412-424 month unknown. .
Guckel et al., "Fabrication of Assembled Micromechanical Components
Via Deep X-Ray Lithography," IEEE 1991, pp. 74-79 month unknown.
.
McCormick et al., "Microengineering design and manufacture using
the LIGA process," Engineering Science and Education Journal, Dec.
1994, pp. 255-262. .
Kenny et al., "A Micromachined Silicon Electron Tunneling Sensor,"
IEEE 1990, pp. 192-196 month unknown. .
MacDonald, "Single Crystal Silicon Nanomechanisms for Scanned-Probe
Device Arrays," IEEE 1992, pp. 1-5 month unknown. .
"51st Annual Device Research Conference," IEEE Transactions on
Electron Devices, vol. 40, No. 11, Nov. 1993, pp. 2098-2099. .
Judy et al., "Magnetic Microactuation of Polysilicon Flexure
Structures," 1994 Solid-State Sensor and Actuator Workshop (Hilton
Head, SC), Jun. 1994 ..
|
Primary Examiner: Stephan; Steven L.
Assistant Examiner: Jones; Judson H.
Attorney, Agent or Firm: Croll; Timothy Rex
Claims
We claim:
1. A micro-mover, comprising:
two closely spaced silicon plates, at least one of said plates
including integral suspension springs that support a moving plate
portion; and
means for driving said moving plate portion by planar
electromagnetic actuation so as to effect a reciprocal movement of
said two plates relative to each other in both an X-direction and a
Y-direction, said Y-direction being substantially transverse to
said X-direction.
2. The micro-mover of claim 1, comprising:
two substantially identical, closely spaced silicon plates, wherein
a first of said plates is adapted for movement in an X-direction,
and wherein a second of said plates is adapted for movement in a
Y-direction.
3. The micro-mover of claim 1, comprising:
a first, stationary silicon plate; and
a second silicon plate comprising compound springs that are adapted
to allow said plate to move in both an X-direction and a Y-
direction, relative to said first, stationary plate.
4. The micro-mover of claim 1, wherein said integral springs
comprise:
a symmetric folded spring configuration, including a spring formed
at each corner or on at least two sides of said moving plate.
5. The micro-mover of claim 1, comprising at least two silicon
plates that are assembled facing each other, closely spaced and
parallel, having axes of motion at 90.degree. apart.
6. The micro-mover of claim 1, wherein said moving plate provides a
surface upon which a recording medium and/or a read/write device
may be placed.
7. The micro-mover of claim 1, said means for driving further
comprising:
at least one of an X-axis multipole magnet, a Y-axis multipole
magnet, and a combination X-axis/Y-axis multipole magnet.
8. The micro-mover of claim 7, said multipole magnet further
comprising: a flux return plate.
9. The micro-mover of claim 1, further comprising: one or more
covers for hermetically enclosing said micro-mover.
10. The micro-mover of claim 9, said one or more covers further
comprising: one or more spacing posts; and a seal gasket.
11. The micro-mover of claim 1, said plate further comprising: one
or more contact pads and associated electrical leads.
12. The micro-mover of claim 7, said means for driving further
comprising:
at least one drive coil for effecting plate motion, wherein current
in said drive coil reacts with a magnetic field developed by said
magnet to produce a force that moves said plate against the force
of said plate suspension springs.
13. The micro-mover of claim 12, wherein said drive coil is a
symmetric coil.
14. The micro-mover of claim 12, wherein said drive coil is an
asymmetric coil.
15. The micro-mover of claim 7, said magnet further comprising:
alternating magnetic poles magnetized into a monolithic magnet
structure.
16. The micro-mover of claim 1, said means for driving further
comprising:
at least one torque coil for producing a correction torque to
cancel unwanted torque produced by said drive coil.
17. The micro-mover of claim 1, further comprising:
at least one capacitance position sensor, adapted to change
capacitance with relative parallel plate motion, and for producing
an output signal in accordance therewith, wherein said position
sensor is insensitive to motion in an orthogonal axis and relative
plate spacing.
18. The micro-mover of claim 17, said capacitance position sensor
further comprising:
three capacitor plates forming two capacitors, comprising a first
and a second overlapping capacitor formed on two of said mover
plates, wherein relative positive movement of one of said two mover
plates increases said first capacitor's capacitance and decreases
said second capacitor's capacitance, and wherein relative negative
movement of the same mover plate increases said second capacitor's
capacitance and decreases said first capacitor's capacitance.
19. The micro-mover of claim 18, further comprising:
a first and a second timer circuit, said timer circuits
respectively connected to said first and second capacitors, such
that when the capacitance of a capacitor associated with a timer is
changed, the frequency output of said timer changes; and
a third capacitor/third timer for canceling effects of stray
capacitance that has the same stray capacitance as said first and
second capacitors but no moving plates.
20. The micro-mover of claim 1, further comprising:
at least one Z-capacitor for controlling and maintaining plate
spacing.
21. The micro-mover of claim 11, wherein said electrical leads are
routed over said springs to said moving plate on either one or both
sides of said plate.
22. The micro-mover of claim 11, further comprising:
an umbilical connector, including leads formed on a flexible
insulator that spans a gap between said moving plate and a
non-moving portion of said plate.
23. A micro-mover, comprising:
at least two etched single crystal silicon plates, at least one of
said plates including a plate suspension system that facilitates
relative plate motion with high resolution, while preventing
changes in plate spacing due to such effects as external
acceleration, temperature, humidity, and aging; and
an actuator that moves said plates in response to electrical
signals, while exhibiting only minimal cross-talk between
respective axes of plate motion.
24. A micro-mover, comprising:
at least one etched single crystal silicon plate, said plate
including integral suspension springs that support a moving plate
portion, said suspension springs having a symmetrical double folded
spring configuration, including a spring formed at each corner or
on each side of said moving plate;
means for driving said moving plate portion by planar
electromagnetic actuation, including at least one of an X-axis
multipole magnet, a Y-axis multipole magnet, and an X-axis/y-axis
multipole magnet, at least one drive coil for effecting plate
motion, wherein current in said drive coil reacts with a magnetic
field developed by said magnet to produce a force that moves said
plate against the force of said plate suspension springs, at least
one torque coil for producing a correction torque to cancel
unwanted torque produced by said drive coil, and a flux return
plate;
at least one capacitance position sensor, adapted to change
capacitance with relative parallel plate motion, and for producing
an output signal in accordance therewith;
at least one Z-capacitor for controlling and maintaining plate
spacing;
one or more contact pads and associated electrical leads; and
one or more covers for hermetically enclosing said micro-mover, at
least one of said covers including one or more spacing posts, and a
seal gasket.
25. The micro-mover of claim 24, comprising:
two substantially identical, closely spaced silicon plates, wherein
a first of said plates is adapted for movement in an X-direction,
and wherein a second of said plates is adapted for movement in a
Y-direction, wherein said two plates are assembled facing each
other, closely spaced and parallel, having axes of motion at
90.degree. apart.
26. The micro-mover of claim 24, further comprising:
a first, stationary silicon plate; and
a second silicon plate comprising compound springs that are adapted
to allow said plate to move in both an X-direction and a Y-
direction, opposite said first, stationary plate.
Description
BACKGROUND OF THE INVENTION
Technical Field
The invention relates to actuators. More particularly, the
invention relates to micro-mover actuators.
Description of the Prior Art
A memory device, using two parallel plates that carry read-write
transducers on one plate and a recording medium on the other plate,
requires a micro-mover to move its elements orthogonally relative
to one another in-plane, and to control the spacing between the
elements. The object of such device is to move an array of
read-write elements on one plate to scan a recording medium on the
opposing plate, such that the combined relative in-plane motion of
the two plates facilitates data transfer to and from the recording
medium. Such motion enables each element to access an area equal to
the product of the relative X- and Y-motion. Accurate spacing
control between the two plates is necessary to enable
non-contacting motion, while maintaining the proximity of the
plates required to enable data transfer.
Typically, this can be accomplished by an approximately 6 mm square
silicon plate that is moved relative to a similar plate in three
orthogonal directions with a relative in-plane motion of about 100
.mu.m in the X and Y directions, and whose spacing Z must be
adjustable in a range of about 0.4 .mu.m. The X and Y travel must
be continuous with a resolution of about 20 nm while the Z travel
requires a resolution of about 10 nm over the entire recording
area. Using silicon, a plate of 300-400 .mu.m thickness appears
essential to maintain adequate flatness. In such an application, a
400 .mu.m thick plate with a moving mass of 35 mg must be
accelerated at more than 10 G's (100 M/s.sup.2) to achieve an
access time of 1 msec. The ability to respond to external
perturbations may require more than 50 G's of acceleration. The
minimum driving force required for a 50 G acceleration is 17.5 mN.
Other designs where spring forces become dominant may require
forces greater than 40 mN. However, peak power to achieve this
motion must remain low, specifically below 1 W for the example just
described.
A number of micro-mover actuators have been proposed. The most
common actuator for micro-movers uses a capacitive electric comb in
which arrays of conducting fingers of opposing polarity are
interleaved. The fingers are drawn into greater engagement when a
voltage is applied to them. The force generated by this approach,
in SI units, is given by the equation: ##EQU1## where 2 .mu..sub.0
c.sup.2 =2.22.times.10.sup.11, W is the width of the fingers
perpendicular to the motion, N is the number of spaces, V is the
voltage, and d is the spacing between the fingers. The principal
drawback to this approach is the limited amount of force that is
achievable for practical voltages V and spacing d. Assuming the
following, V=100 volts, N=100 spaces, W=20 .mu.m, and d=1 .mu.m,
the resulting force is 0.09 mN (9.0.times.10.sup.-5 N). This force
is 500 times too small for the application described above. It is
not currently feasible to produce a W/d ratio greater than 20:1
except with LIGA (see M. McCormick, E. Chowanietz, A. Lees,
Microengineering Design and Manufacture Using the LIGA Process,
IEEE Engineering Science and Education Journal, pp. 255-262,
December 1994), which is impractical for single crystal silicon.
Electric fields V/d exceeding about 1.times.10.sub.8 V/m, as in
this example, are not practical.
Other known micro-actuators include:
A piezo-electric transducer that expands when a voltage is applied
to it. A piezo motor can produce large forces with small motion,
but has excessive hysteresis, responds too slowly, and the size
required to produce sufficient motion is too large;
A magnetostrictive motor that when magnetized expands in length.
However, such an actuator requires excessive length to achieve
adequate travel, even with large magnetostrictive materials such as
terbium iron, TbFe.sub.2, in which saturation magnetostriction can
achieve over 2000 ppm length changes; and
A micro-actuator, referred to as an inchworm, uses either a piezo
or magnetostrictive element. Such actuators generate many small
steps to produce larger motion. Their drawback is the use of
friction clamps that are not repeatable and whose sliding surfaces
wear and generate unacceptable debris.
Conventional linear electromagnetic motors are also known that
generate large attractive forces in the Z direction, between the
parts moving in the X- or Y-direction, thereby producing excessive
relative Z-motion. Because sliding bearings wear, cause wear
debris, and have excessive friction, they cannot be used to prevent
Z-motion. Ball bearings have excessive runout and hysteresis for
these small motions.
Thermal expansion or phase change motors present problems similar
to piezo and magnetostriction motors, including insufficient travel
for practical size. Phase change motors use temperature induced
phase change in a material to produce motion that is not adequately
controllable because it is hysteretic and nonlinear with
temperature.
Stages with sliding or rolling bearings cannot be used to support
the two plates because they have excessive friction, wear,
hysteresis, and backlash in their mechanisms. Previous designs have
been unable to achieve high forces while maintaining the small
package, light weight, low power consumption, low hysteresis, and
high resolution necessary in a mover for such applications as
parallel plate memory devices. Reducing plate mass by thinning or
hollowing regions would increase distortions caused by external
shock and internal stress, thereby compromising flatness. It would
therefore be desirable to provide a mover constructed of a solid
thick plate having relatively high mass and one that achieves
enough force, e.g. about 40 mN, to accelerate plates that are
sufficiently thick to maintain required dimensional stability over
a large recording area, e.g. 5.times.5 mm.
SUMMARY OF THE INVENTION
The invention provides a forceful, compact magnetic micro-mover
that includes at least one etched single crystal silicon plate
having integral springs that provide a stable, low internal stress.
Although discussed above in the context of a storage device having
two or more moving plates, the described mechanism is applicable to
numerous other applications.
The micro-mover includes a structure of springs etched in silicon
that support a rectangular plate which, in the preferred embodiment
of the invention, is about 6.times.6 mm. The plate is driven by
planar electromagnetic actuation. While etched silicon spring
structures are known (see R. A. Brennan, M. G. Lim, A. P. Pisano,
A. T. Chou, Large Displacement Linear Actuator, Technical Digest,
IEEE Solid-State Sensor and Actuator Workshop, pp. 135-139 (1990);
N. Takeshima, K. J, Gabriel, M. Ozaki, J. Takehashi, H. Horiguchi,
H. Fujita, Electrostatic Parallelogram Actuators, Transducers '91,
1991 International Conference on Solid-State Sensors and Actuators,
Digest of Technical Papers, pp. 63-66 (1991); T. Hirano, T.
Furuhata, K. J. Gabriel, H. Fujita, Design, Fabrication, and
Operation of Submicron Gap Comb-Drive Microactuators, J.
Microelectromechanical Systems, Vol. 1, No. 1, pp. 52-59 (1992);
and S. Johansson, J. Schweitz, Fracture Testing of Silicon
Microelements in situ in a Scanning Electron Microscope, J. Appl.
Phys. 63 (10), pp. 4799-4803 (1988)), such structures have
heretofore had insufficient travel and/or insufficient Z-stiffness.
The invention herein provides tall, narrow, relatively compact
springs that are specifically designed to exhibit the required
lateral travel and high vertical stiffness. The micro-mover thus
consists of a specific etched silicon spring in combination with a
planar electromagnetic (i.e. voice coil) actuator that is capable
of generating large forces (>40 mN), while dissipating less than
1 W peak power.
The micro-mover also includes a plate suspension system that
facilitates the required plate motions and resolution, while
preventing changes in plate spacing from effects such as external
acceleration, temperature, humidity, and aging; and includes an
actuator that appropriately moves the plates in response to
electrical signals, such that there is only minimal cross-talk
between axes of motion.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a single crystal silicon micro-mover
according to a preferred embodiment of the invention;
FIG. 2 is an exploded view of a micro-mover according to the
preferred embodiment of the invention;
FIG. 3 is a plan view of a compound micro-mover according to an
equally preferred embodiment of the invention;
FIG. 4 is a detail perspective view of the (111) and (411) planes
of a plate for a micro-mover that is fabricated from a single
crystal material according to the invention;
FIG. 5 is a sectioned schematic view of a linear motor design for a
micro-mover according to the invention;
FIG. 6 is a plan view of a symmetric driver coil and torque coil
for a micro-mover according to the invention;
FIG. 7 is a plan view of an asymmetric driver coil and torque coil
for a micro-mover according to the invention;
FIG. 8 is a sectioned schematic view showing torque compensation
for a micro-mover according to the invention;
FIG. 9a is a partial schematic diagram/perspective view of a
capacitance bridge position sensor according to a preferred
embodiment of the invention;
FIG. 9b is a schematic diagram of the capacitance bridge position
sensor of FIG. 9a;
FIG. 10a is a partial schematic diagram/perspective view of a
capacitance timer position sensor according to an alternative,
equally preferred embodiment of the invention;
FIG. 10b is a schematic diagram of the capacitance timer position
sensor of FIG. 10a;
FIG. 10c is a graph plotting signal strength versus plate position
for a micro-mover that incorporates the capacitance timer position
sensor of FIG. 10a;
FIG. 11 is a partial schematic diagram/top plan view of a
capacitance bridge position sensor according to another, equally
preferred embodiment of the invention;
FIG. 12 is a perspective view of a capacitor structure that is used
to generate Z forces in a micro-mover according to the
invention;
FIG. 13 is a graph plotting capacitor and spring force as functions
of capacitor separation and voltage for four 1 mm.sup.2 capacitors
formed between opposing micro-mover plates according to the
invention;
FIG. 14 is a graph plotting capacitor separation of opposing
micro-mover plates as a function of capacitor voltage for four 1
mm.sup.2 capacitors formed between said plates according to the
invention;
FIG. 15 is a graph plotting restoring force versus differential
Z-displacement for opposed micro-mover plates at the operating
equilibrium between restoring springs and four 1 mm.sup.2
capacitors held at 86 V according to the invention;
FIG. 16 is a perspective view of symmetrical Z-force capacitor
structures for a micro-mover according to the invention; and
FIG. 17 is a perspective view of an umbilical cable for a plate in
a micro-mover according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
This invention consists of an etched single crystal silicon
spring-and-plate design that is moved by an electromagnetic
actuator incorporated into the mover. Two preferred micro-mover
configurations are described herein, although the invention is
readily applied to other configurations, depending upon its use. A
first preferred configuration (see FIGS. 1 and 2) has two
essentially identical, closely spaced plates, wherein one plate is
adapted for movement in the X-direction and the other plate is
adapted for movement in the Y-direction. A second, equally
preferred configuration (see FIG. 3) has compound springs that are
adapted to allow one plate to move in both the X- and Y-
directions, opposite a second stationary plate, that is not
shown.
Two significant novel design features of the micro-mover herein
described include:
A plate suspension that facilitates the required relative plate
motions and resolution, while preventing changes in plate spacing
due to such effects as external acceleration, internal stress,
temperature, humidity, and aging; and
An electromagnetic drive that appropriately moves the plates in
response to electrical signals, with minimal cross-talk between the
axes of motion.
FIG. 1 is a top plan view of a single crystal silicon micro-mover
according to the first preferred embodiment of the invention. The
figure shows one of two plates 10 of the micro-mover, in which a
moving plate 12 is held in suspension relative to an outer frame 15
by several silicon support springs 13. Relative motion of the plate
12 is indicated by the arrow that is identified by the numeric
designator 19.
Orientation of the wafer from which the plate 10 is fabricated is
indicated in the figure by the numeric designator 11, which shows a
<110> wafer flat for a (100) wafer, where "<110>"
denotes a 110 crystallographic direction and "(100)" denotes a 100
crystallographic plane. The (111) silicon planes 14 of the wafer
and the (100) silicon plane perpendicular to the plate plane 17 are
also shown in the figure. The actual spring thickness Ws may be on
the order of 40 .mu.m, while the etch gap Wb where silicon is
removed 18 may be on the order of 470 .mu.m.
The exemplary micro-mover includes four springs formed at each
corner of the moving plate. This spring configuration is a
symmetric double folded spring. When a plate is moved to an off
center position, the springs are each bent into an "S" shape. This
shortens the horizontal projected length of each spring. If a
single non-folded spring were used at each corner of the plate,
plate motion would require both bending and axial lengthening of
each spring. This axial lengthening (tensioning) produces a
non-linear Y spring constant that is not compatible with the
requirements of a large Y travel, a large ratio of Z stiffness to Y
stiffness, and a compact spring design. Axial shortening can be
accommodated by using a folded spring design. However, a single
folded spring does not provided a large ratio of Z stiffness to Y
stiffness. This problem is solved by using a symmetrical double
folded spring design as shown in FIG. 1. This spring design has a
linear Y spring constant and a large ratio of Z stiffness to Y
stiffness (>100).
FIG. 2 is an exploded view of a micro-mover 20 according to the
first preferred embodiment of the invention. Two of the plates 10a,
10b described above in connection with FIG. 1 are assembled facing
each other, closely spaced and parallel, with their axes of motion
orthogonal to one another. Each plate includes a complement of
silicon springs 13a, 13b that hold a corresponding moving plate
12a, 12b in suspension. Each moving plate provides a surface on
which a recording medium or read/write device may be placed.
The micro-mover also includes an X-axis multipole magnet 22a and a
Y-axis multipole magnet 22b. Each magnet includes a flux return
plate 30 (not shown for the moving plate 10b). The micro-mover
assembly is preferably hermetically enclosed in a pair of covers 31
(not shown for the moving plate 10b) which include one or more
spacing posts 28 and a seal gasket 29. The two plates also each
include one or more contact pads 24a, 24b and associated electrical
leads 25a, 25b, a drive coil (of which the Y-axis drive coil 23 for
the Y-axis plate is shown in the figure), a torque coil (of which
the Y-axis torque coil 21 for the Y-axis plate is shown in the
figure), a capacitance bridge position sensor (of which the X-axis
capacitance bridge sensor 26 for the X-axis plate is shown in the
figure), and one or more Z-capacitors for spacing control. One
plate of each Z-capacitor 27 is on the top of the x-axis plate, and
the other plate of each Z-capacitor is on the underside of the
y-axis plate (not shown in FIG. 2).
One advantage of the embodiment of the invention shown in FIGS. 1
and 2 is that both plates can have nearly identical masses and
Z-motion spring constants, such that Z-spacing variation may be
minimized by common mode response to Z-perturbations. Thus, control
of Z-spacing to a desired tolerance is less difficult to achieve.
In contrast, the embodiment of the invention shown in FIG. 3 has
the advantage of simplified assembly, using a single large magnet
and a fixed plate that facilitates interconnect. However, the
embodiment of FIG. 3 does not have common mode rejection of
Z-spacing variations that are due to Z-axis shocks. In both
embodiments of the invention, read-write elements are located on
the surface of one plate with the recording medium on the face of
an opposing plate as shown in FIG. 2.
In embodiment of the invention shown in FIGS. 1 and 2, the drive
coils 23 can be formed by electroplating, and are preferably
located on the outer facing surfaces of the moving plates. In FIG.
2, the drive coil 23 is shown adjacent to a multipole magnet 22b,
which produces fields by which current in the drive coil generates
a drive force to move the plate 12b.
Current in the drive coils reacts with the field of the permanent
magnet to produce a Lorenz force that moves the plate against the
force of its suspension springs. In a data storage application, one
plate may oscillate in the X-direction at or near resonance, for
example at about 1000 Hz. This linear motion corresponds to
rotation of a disk in a conventional computer storage disk, and is
preferably designed to have a high-Q motion and thereby save power,
where Q is the ratio of energy stored per cycle to energy
dissipated per cycle.
However, the Q for single crystal silicon structures can be greater
than 5000 and may present control problems. To control the Q, an
eddy current damping film on the coil side of the plate (not
shown), a shunt resistance across the coil (not shown), or both can
be used to reduce the Q to about 200. The Q of the opposing plate,
moving in the Y-direction, which is analogous to the track seek
operation of a conventional disk drive in a data storage
application, can also be so limited for better servo position
control.
The required length of X- and Y-travel arises from the size of the
recording area and the density of the transducer grid in which each
transducer accesses a small rectangular area. The Y- axis (i.e.
seek axis in a data storage application) must be capable of moving
to, and holding position at the extremes of the data block
associated with each transducer of the array. However, if the
resonant amplitude of the X-axis (i.e. scan axis in a data storage
application) is limited to the length of the data block, the
velocity at the maximum excursion would be zero. To reduce velocity
variation of the data scan, the stroke must be longer than the data
block. For example, if the stroke were 1.8 times the length of the
data block, velocity variation within the data block would be about
20%, as would the bit spacing of data written at constant
frequency.
FIG. 3 is a plan view of a compound micro-mover 35 according to an
equally preferred embodiment of the invention, in which one moving
plate 37 moves in the both X- and Y-directions. The plate is
fixedly suspended for Y-axis motion relative to an intermediate,
moving frame 36 by a set of Y-axis springs 40; and is fixedly
suspended for X-axis motion relative to an outer, non-moving frame
39 by a set of X-axis springs 38.
To generate X-motion in this embodiment, a force is produced on the
moving plate in the X-direction by a drive coil (not shown on FIG.
3) that is formed on the bottom of the plate, while Y-motion is
achieved by producing a force in that direction from one or more
drive coils located on either the intermediate frame 36, or on the
moving plate 37. The multipole magnet (not shown on FIG. 3) for
X-motion and for Y-motion can be combined into a single magnet
having poles that are magnetized in two different orientations.
This simplifies assembly and reduces the cost of manufacture. Two
advantages of this embodiment are that the non-moving plate does
not require flexible electrical connections to the stationary frame
39, and that it has fewer parts. One disadvantage of this
embodiment, relative to that of FIGS. 1 and 2, is a lack of common
mode rejection for mechanical shock that can cause Z-spacing change
between the movable and stationary plates. It is presently thought
that the embodiment of FIGS. 1 and 2 is preferred for data storage
applications because the control of Z-spacing between read-write
elements and the recording medium is a dominating concern.
Required Forces
The micro-mover must provide adequate force to overcome spring
forces and the forces of accelerating and moving the mass to
achieve the specified access time. For rotating disk storage,
access time is composed of latency, i.e. rotational period, and
seek time, i.e. time to move transducer between tracks. In the
micro-mover device herein described, the equivalent of latency is
the period of the resonant axis, if data must be read in the same
direction as written. The time required to move between track
maxima is the maximum seek time. Latency and maximum seek time for
the exemplary micro-mover are specified as 1 msec. Stroke lengths
for the mover are determined by the density of read-write
transducers and the total recording area.
The minimum area required for the moving plate is determined by
dividing the specified data capacity of the storage device by the
achievable recording density of the storage medium. Additionally,
area must also be added to the moving plate to accommodate sensors,
circuitry, and interconnect. A 6 mm square silicon plate has been
selected as a representative example. For such plate, the Z-spacing
specification might require that the surface of the moving plates
be flat to within 10 nm over the recording area (5.times.5 mm
minimum). Although single crystal silicon has inherently low
internal stress, stresses expected to be introduced during
processing of the medium and read-write transducers require
substantial plate stiffness. A thickness of at least 300 .mu.m is
considered essential for adequate flatness in the presence of
static, e.g. internal stresses, and dynamic perturbations, such as
Z-acceleration. For this example, a conservative plate thickness of
400 .mu.m with a mass of 35 mg has been chosen.
For a storage device having 4096 transducers, e.g. arranged in a
64.times.64 array, over a 5 mm square recording area, the minimum
travel required is .+-.39 .mu.m. With 10000 transducers, e.g.
arranged in a 100.times.100 array, the minimum travel would be
.+-.25 .mu.m. The stroke required for the exemplary micro-mover is
assumed to be .+-.39 .mu.m in the seek direction. The resonant
direction requires greater travel to reduce velocity variation
through the data zone. To limit velocity variation to 20%, travel
in the resonant direction must be 1.8 times as large as the data
block or .+-.70 .mu.m.
For the micro-mover resonant axis, a 1 msec latency requires a
resonant frequency of at least 1000 Hz. Assuming a linear
spring-mass model, the required spring rate is given by:
For a 35 mg (m=3.5.times.10.sub.-5 kg) moving mass and a 1000 Hz
resonant frequency f, the spring rate required is 1.38 mN/.mu.m.
For a micro-mover in accordance with the embodiment of FIGS. 1 and
2, having identical resonant and seek axes, the minimum force
required to seek a distance of .+-.39 .mu.m is 1.38.times.39, or 54
mN (5.5 gf). Because transit time for limit-to-limit travel is only
0.5 msec, i.e. half the resonant period, the minimum required force
is determined by the resonance requirement rather than by the
moving plate inertia.
Although inertia does not dominate the force requirement, it is
calculated here for purpose of comparison. The velocity profile
requiring the smallest force is triangular, i.e. a maximum positive
acceleration during the first half of the stroke and the reverse of
the remainder of the stroke. Using a triangular profile and
ignoring spring forces, the acceleration and force required to move
a 35 mg mass over a distance of 78 .mu.m in 1 msec are 78 m/s.sup.2
(.about.8 G) and 2.73 mN, respectively. The maximum velocity
achieved during the move is 0.04 m/sec. For this example, the
minimum force required to overcome the springs is about 20
(54/2.73) times greater than the force required to accelerate the
inertia. This ratio depends on the resonant frequency, travel
distance, and transit time, but not on the mass. However, reducing
the mass linearly decreases the force required to overcome the
springs.
Fabrication of Plate and Springs
FIG. 4 is a detail perspective view of the (111) and (411) planes
of a portion 40 of the plate 10 (FIG. 1) for a micro-mover that is
fabricated from a single crystal material according to the
invention. In the preferred fabrication method, the moving plate
and springs shown in the figure are formed by two-sided etching of
single crystal silicon (100) wafer that has been polished on both
sides. The mask pattern is transferred by standard lithographic
methods to an etch resistant layer on both sides of the silicon
wafer. The pattern is oriented on the wafer with the mask openings
parallel to the (100) planes (45.degree. to the standard
<110> flat) with the front and back images superimposed. An
anisotropic etchant such as 30-50 w % KOH in water at
60-100.degree. C., together with an etch resistant layer of low
stress CVD SiN is preferred. Etching is continued beyond
breakthrough of front and back etch fronts to achieve required
spring widths.
At breakthrough, a series of slots through the wafer, defined by
(100) planes in the long axis and terminated by four (111) planes
at their ends will have been formed. The (111) planes make an angle
of 54.7.degree. with the (100) plane of the wafer surface, as well
as the slot sides. When a slot makes a right angle turn, two (111)
planes define the outside of the turn and a more complex
combination of (411) and (other) planes define the inside. FIG. 4
shows a close-up view of the (111) planes 14 and the (411) planes
42. The position of the intersections between (111) planes and the
wafer surface are defined by the corners of the mask openings.
Prior to breakthrough, and far from the slot ends, the surfaces of
the etch groove are formed predominately by three (100) planes that
etch at the same rate. Each of the two bottom corners are formed by
two small (210) planes. The slot widens twice as fast as it
deepens, and because the etch is two sided, the breakthrough width
Wb of the slot is given by:
where W.sub.m is the width of the mask opening and t is the wafer
thickness. The spring widths Ws at breakthrough are given by:
where w is the distance between the centers of the two mask
openings in the spring region.
Because final spring widths depend on wafer thickness, mask design
must allow for the distribution of this thickness. Controlling
final spring dimensions may require end point detection after
initial breakthrough. Thickness uniformity both among wafers, and
within the same wafer, are important parameters in controlling
dimensions.
At breakthrough, top and bottom pairs of (111) planes at the ends
of the slots meet at an edge midway through the wafer. As etching
continues beyond breakthrough, the edge inverts, i.e. it becomes
reentrant, exposing two more (111) planes. Two (111) intersections
are formed that invert in a similar fashion.
This etching technique readily produces springs that are 400 .mu.m
deep (wafer thickness t) and 40 .mu.m or less wide (W.sub.s), such
that aspect ratios greater than 10:1 are attainable. A large aspect
ratio is important for springs that are compliant in the direction
of motion but stiff in the direction perpendicular to the plane of
the moving plate. In the direction of motion, the spring constant
varies as the third power of W.sub.s and varies linearly with t.
Whereas, the spring constant in the Z-direction varies as the third
power of t and varies linearly with W.sub.s. The ratio of spring
constants is thus: ##EQU2## If t is 400 .mu.m and W.sub.s is 40
.mu.m, then K.sub.z /K.sub.y is equal to 100.
Fracture Strength of Silicon
Fracture strength and fatigue characteristics of single crystal
silicon are critical to the reliable operation of the silicon
spring structure described above. Theoretical cohesive strength of
single crystal silicon is approximately 30 GPa depending on its
crystalline orientation. Experimentally determined fracture
strengths reported in the literature (see Johansson et al. ibid.;
and K. Yasutake, M. Iwata, K Yoshii, M. Umeno, H. Kawabe, Crack
Healing and Fracture Strength of Silicon Crystals, J. Mater. Sci.,
21, pp. 2185-2192 (1986)) range from 50 MPa to 10 GPa, depending on
such factors as test geometry, method of fabrication, and sample
size. This variation arises from the intrinsic fracture mechanics
of single crystal silicon, where strength is determined
predominately by surface and near surface defects. Because the
probability of critical defects is reduced for small structures,
such structures can more closely approach theoretical strength. The
fabrication method is important to controlling defects and
micro-geometry that affect fracture strength.
A simplified spring and plate geometry was analyzed by finite
element analysis that predicted a maximum von-Mises stress of 122
MPa at a displacement of 70 .mu.m. Maximum stress in the actual
structure is higher because the stress concentration caused by the
intersection of the two (111) planes with the (100) side wall at
the root of each spring was not included in the model. Nonetheless,
this value is at the low end of observed silicon fracture
strength.
Fracture strength depends upon part geometry, test geometry, and
the load at fracture. For the micro-mover herein described,
displacement at fracture is the primary concern. Measurements were
made of the displacement at which fracture occurred using resonant
AC excitation. Fracture occurred form displacements of from 180 to
200 .mu.m. This gives a safety factor of between 2.5 and 3.0. The
lower bound for fracture strength based on the simplified finite
element model and the observed fracture displacement is about 300
MPa.
Actuator Design
FIG. 5 is a sectioned schematic view of a linear actuator design 50
for an electromagnetic micro-mover according to the invention.
Alternating magnetic poles, preferably magnetized into a monolithic
magnet 51 that includes an associated flux closure plate 53,
produce the external fields 56 schematically shown in the figure.
Conductors 54, located in a plane parallel to the magnet 51, are
arranged so that each group is centered adjacent to a magnet pole
52. Because the current direction is opposite in adjacent groups,
as are the polarities of the magnet poles, they produce a
unidirectional force.
The net force 55 applied to the moving plate 58 is parallel to the
plate and through the conductor centers. Motion is limited to the
difference between the coil bundle width W.sub.CB and the magnet
pole width Wp. In practice, the difference, W.sub.D =W.sub.p
-W.sub.CB, is larger than the required travel to minimize
nonlinearity. The selection of W.sub.D is a compromise between
actuator linearity and efficiency. If high energy product magnets
are used, such as NdFeB, and if the coil conductors are spaced
closely to the pole surfaces, then substantial forces can be
produced.
The magnetic force in SI units is given by:
where B is the magnetic field around the conductors, L is the
length of the conductors, I is the current, and N is the number of
conductors in the field. Typical values are B=0.6 Tesla, L=0.005 m,
I=0.2 Amp, and N=100. These values produce a force of F=60 mN.
As the pole width Wp is decreased, the required size and field
range of the magnet is also decreased. A ratio of 1:2 for pole
length (or thickness, in the case of a monolithic magnet) divided
by pole width, is readily feasible with NdFeB or CoSm magnets. The
magnet required for the micro-meter should be thinner than the
mover thickness, preferably 300 .mu.m or less. For a 300 .mu.m
thick magnet, pole width should be no larger than 600 .mu.m.
Optimal selection of pole width is based on the following partial
list of factors: efficiency, required field extent, required magnet
size, magnet energy product, and magnet-to-coil spacing.
Drive Coils
FIG. 6 is a plan view of a symmetric drive coil 23a for a
micro-mover plate 12 according to the invention. The plate 12 also
includes a torque coil 21a (discussed below). The current, supplied
to the coil through the electrical leads 60, in alternate bundles
of conductors flows in opposite directions in the coil 23a. The
embodiment of the invention shown in FIGS. 1 and 2, which has two
moving plates, uses this design for both axes. Motion of the plate
shown in the figure is indicated by the arrow identified by the
numeric designator 61.
FIG. 7 is a top plan view of an asymmetric driver coil 23b for a
micro-mover according to the invention, whose efficiency is
equivalent to the symmetrical design, if it is offset to make its
force act through the center of the plate. Current is supplied to
the coil 23b through electrical leads 70. Motion of the moving
plate 12 shown in the figure is indicated by the arrow identified
by the numeric designator 71. The plate 12 also includes a torque
coil 21b (discussed below).
Torque Coil
FIG. 8 is a sectioned schematic view showing torque compensation
for a micro-mover according to the invention. The force 55 produced
by the magnetic field 56 shown in the figure is applied to coils on
the plate 58 surface that are a half plate thickness offset from
center of mass, and to its spring attachments that also lie on the
central plane of the plate. This offset causes an unwanted torque
for which a torque compensating coil 21 is provided.
The torque coil 21 and torque magnet poles are spaced so that the
coil resides in a magnetic field 56a that is oriented parallel to
the motion, with a polarity that gives a resulting force F1 (81)
upward on the left of the torque coil and downward F2 (82) on the
right, or the converse when the current and motion is reversed. The
forces F1 and F2 produce a correction torque 80 that can be
adjusted to cancel unwanted torque by the offset of the drive coil
by choosing an appropriate ratio and polarity to the current in the
drive coil. The shape and location of the conductors connecting the
active runs of the torque coil is unimportant. Various connecting
conductor arrangements can be used in the scope of this invention.
The region 83 of the magnet is not significantly magnetized so the
required field 56A can be obtained
FIG. 6 shows a torque correction coil 21a, and leads 91 therefor,
for a symmetric driver coil 23a in a micro-mover according to the
invention; and FIG. 7 shows a torque coil 21b for an asymmetric
driver coil 23b, and leads 101 therefor, in a micro-mover according
to the invention.
Magnets
A multi-pole monolithic magnet is the preferred design for the
micro-mover herein described. The required magnet pole structure
exceeds the capabilities of standard approaches to magnetizing
multi-pole magnets. Two types of magnetizing structures may be used
to magnetize the desired pole structure into the magnet, both of
which are two pole designs. In the first design, single poles and
single turn drive coils are located on opposite faces of the
magnet. Poles are written one-at-a-time through the material with
alternating polarity. The step size is adjusted when writing the
outside poles to produce the neutral gaps required for the torque
coil. This method has produced a pole structure with 300 .mu.m
pitch in a 300 .mu.m thick magnet.
An alternative magnetizing structure locates two 290 .mu.m poles
side-by-side on the same side of the magnet with a gap of 20 .mu.m,
such that two magnet poles at a time are written into the material.
A second pole structure, having a wider spacing, must be used to
write the poles of the torque compensation coil. The advantage of
this second approach is that the magnet does not require a
magnetically soft flux return plate. The disadvantage of this
approach is that a second pole structure is required to write the
torque compensation poles.
Magnet material selection is governed by energy product,
temperature stability, and machinability. The higher energy product
and demonstrated ability to fabricate magnets to the required size
make NdFeB the preferred material, although other materials, such
as CoSm, may also be used.
X- and Y-Position Sensors
FIG. 9a is a partial schematic diagram/perspective view of a
capacitance bridge position sensor 90 according to a preferred
embodiment of the invention; and FIG. 9b is a schematic diagram of
the capacitance bridge position sensor of FIG. 9a. The relative
position of the mover plate in the track seek Y-direction, or scan
X-direction, can be sensed by a capacitance bridge (C-bridge). The
bridge is designed to change its capacitance only with relative
parallel plate motion contributing to the output signal, while
Z-changes are ignored. Thus, the capacitances C1 (91) and C4 (95)
increase when the capacitances C2 (92) and C3 (93) decrease for a
particular plate motion, as shown on the figure by the arrow
identified by the numeric designator 98. This arrangement maximizes
the signal to noise ratio of the X- or Y-position signal. The
capacitance bridge is driven by a signal source 96. A differential
capacitance value is provided to an output buffer 97 to produce a
signal that is indicative of relative plate position.
FIG. 10a is a partial schematic diagram/perspective view of a
capacitance position sensor 100 according to an alternative,
equally preferred embodiment of the invention; FIG. 10b is a
schematic diagram of the capacitance position sensor of FIG. 10a;
and FIG. 10c is a graph plotting signal strength versus plate
position for a micro-mover that incorporates the capacitance
position sensor of FIG. 10a. As shown in FIG. 10a, two overlapping
capacitors C1 and C2 are formed on two of the moving plates. When
one plate moves, the capacitance of C1 increases and the
capacitance of C2 decreases, and vice versa. The two capacitors are
connected to two respective timer circuits 105, 106, which may be a
555 timer. Each timer has an associated resistor having a
resistance R. When the capacitance of a capacitor associated with a
timer is changed, the output frequency changes. In the preferred
embodiment of the invention, the period of each output of each
timer is measured, and the periods are then combined.
A third timer 107 is also provided that has the same stray
capacitance as the bridge capacitors 101, 104, but no moving
plates, i.e. it is a dummy capacitor/timer. Instead of measuring
the output frequency, the period of the output oscillation is
measured. Thus, the capacitance of the overlapping plates is
converted to an AC signal by its associated timer. The output
periods T.sub.1 and T.sub.2 of these signals are used because they
vary linearly with plate position y. The third timer is used to
cancel the effects of stray capacitance, as described below. The
periods T.sub.1, T.sub.2, and T.sub.3 are given by:
where R is the timer resistance, K.sub.o is the frequency constant
of the timers, and C.sub.o1, C.sub.o2, and C.sub.o3 are the stray
capacitances of the three timer inputs. The capacitances C.sub.1
and C.sub.2 of the overlapped plates is given in SI units by:
where Y.sub.1 and Y2 are the initial plate overlap distances, Y is
the position change of the two top plates, d is the spacing between
the top and bottom plates, and K is 36*10.sub.9 PI, which is equal
to 1 divided by e.sub.0, the permitivity of free space. The periods
are thus: ##EQU3##
The periods are combined in a unique way to produce a signal S:
##EQU4##
If C.sub.o1, C.sub.o2, and C.sub.o3 are all equal, then:
The signal S is linear in Y with a fixed offset (Y.sub.1
-Y.sub.2)/2. The parameters R, W, K.sub.o, and the distance d have
all canceled out, as well as well as have the stray capacitances.
Thus, a variation in spacing d does not change the signal. This
signal is plotted in FIG. 10c for the example Y.sub.1 =180 .mu.m
and Y.sub.2 =60 .mu.m. This produces a signal:
having a usable range of slightly less than +/-60 .mu.m. At +60
.mu.m, the C.sub.2 overlap goes to 0; and at -60 .mu.m, C.sub.1 and
C.sub.2 become equal and S goes to 0.
For proper operation, the parameters must be constant, or vary the
same with temperature and time. Thus, the three timers should
preferably be on the same silicon chip, and the resistors should be
of identical type, be thermally linked, and be stable with
temperature. The stray capacitances must also track with
temperature and time so they remain equal.
FIG. 11 is a partial schematic diagram/top plan view of a
capacitance bridge position sensor 111 according to another,
equally preferred embodiment of the invention. The bridge
configuration is a variation of the design shown in FIG. 9a, but it
is not as compact. The bridge is driven by a signal source 115 and
the output is sensed by a differential amplifier 114.
Z-spacing Control
FIG. 12 is a perspective view of a capacitor structure 125 that is
used to generate Z forces in a micro-mover according to the
invention. The Z-spacing between the mover plates (only the bottom
plate 120 is shown in the figure) is controlled by means of four
forcing capacitors 126-129. The top capacitor plates are formed on
the bottom of the top moving plate (not shown). The capacitors have
diagonal symmetry and are located on facing surfaces of the mover.
Applying a common voltage to these capacitors produces an
attractive force that pulls the capacitor plates 121, 122, and
therefore the moving plates, closer together with a stroke of up to
0.4 .mu.m against the Z-stiffness of the support springs. When not
energized, the plates pull apart and prevent the read-write
elements from striking the medium in the event mechanical shock. To
the first order, the plates in the embodiment of the inventions
shown in FIGS. 1 and 2 preferably move in common in the Z-direction
maintaining constant spacing. However, because Z-spring constants
cannot be perfectly matched some spacing change may occur. The
Z-capacitors are located in the four corners of the plate in this
design, leaving the central area for recording.
FIG. 13 is a graph plotting capacitor and spring force as functions
of capacitor separation and voltage for four 1 mm.sup.2 capacitors
formed between opposing micro-mover plates according to the
invention. The figure also shows the absolute differential spring
force as the plates are moved from the neutral separation, 3.4
.mu.m in this example. As voltage is applied, the capacitors draw
the plates together until equilibrium is achieved.
FIG. 14 is a graph plotting capacitor separation of opposing
micro-mover plates as a function of capacitor voltage for four 1
mm.sup.2 capacitors formed between said plates according to the
invention. As the voltage is increased, the plates are pulled
closer together n a non-linear manner. At 102.2 volts, the plates
are pulled together irreversibly. The operating point must be well
below 102.2 volts for this example.
FIG. 15 is a graph plotting restoring force versus differential
Z-displacement for opposed micro-mover plates at the operating
equilibrium between restoring springs and four 1 mm.sup.2
capacitors held at 86 V according to the invention. The figure
shows that the spring spacing rate is reduced slightly by the
nonlinear force of the capacitors when they draw the plates
together.
Z-spacing Sensors
The same capacitors used for Z-spacing control, can be used to
sense the spacing between the moving plates. This can be done by
superimposing a high frequency voltage on the control voltage used
to produce the Z-force. Alternatively, a signal from the read-write
elements can be used to sense changes in spacing between the
plates.
Symmetric Z-Capacitors
FIG. 16 is a perspective view of symmetrical Z-force capacitor
structures 161-164 for a micro-mover according to the invention.
The top plate in FIG. 16 is shown cut away to make the bottom plate
more visible. Two of the four capacitors 161, 163 are composed of
solid square plates, one larger plate and one smaller plate. The
other two capacitors 162, 164 are composed of composite plates that
have a large right triangle and a small right triangle connected by
a small link. This design has the correct diagonal symmetry to
allow two identical moving silicon plates to be used together, one
for x motion and one for y motion. In assembly, one mover is
rotated 90.degree. and flipped over, such that the capacitor plates
face each other, as shown in the figure. The top and bottom
capacitor plates are different sizes so the capacitance does not
change as the x or y position changes.
Separation Gasket
FIG. 2 shows a bottom plate 31 having a gasket 29 and a plurality
of posts 28 that control plate spacing and that form a vacuum seal
for a micro-mover according to the invention. A substantially
identical top plate (not shown) is also provided to complete the
micro-mover assembly. Both plates include gaskets that are made
with very nearly equal thickness to space the plates apart the
desired amount, and to allow the plates to be bonded together. The
posts support the assembly to mitigate the force placed on the
assembly through the atmospheric pressure in the center part. In
the preferred embodiment of the invention, the interior of the
assembly is evacuated, and the posts help hold off the force of
atmospheric pressure that tries to squeeze the plates together.
Thus, the silicon frames of the two plates are preferably separated
by a deposited gasket and post design. The gasket material can be a
metal such as copper or gold with a thin indium film to enable
bonding the plates together with accurate parallel spacing for the
moving plates, e.g. for the embodiment of the invention shown in
FIGS. 1 and 2. For this, the gasket material must have suitably
controlled thickness uniformity.
Electrical Conductor Bridges
In FIG. 4, a perspective view of a plate 40 shows electrical leads
on spring structures 13 in a micro-mover according to the
invention. Electrical leads 25 to the drive and torque coils can be
routed over the silicon springs 13 from the non-moving frame 15 to
the moving plate 12. The C-bridge and Z-spacing capacitor leads can
be routed similarly on the opposite side of the springs. Other
electrical leads, such as leads from the read and write elements
can also be routed over springs that are not already occupied by
coil and capacitor conductors. For additional conductors, more
silicon springs could be added if all springs are reduced in
thickness to maintain plate resonant frequency. However, this would
be done at the expense of increased size or reduced recording space
to make room for electronic components.
FIG. 17 is a perspective view of an umbilical connector 172, 173
for a plate 170 in a micro-mover according to the invention. As
shown in the figure, thin leads 172 are deposited on a sheet of
flexible insulator 173, such as Kapton, that spans the gap between
a fixed plate 171 and a moving plate 174 (motion is shown by the
arrow identified by the numeric designator 175). The umbilical
connector must be thin enough not to affect the Q of the moving
plate excessively or cause erratic motion by its spring constant
and/or resonant frequency.
Although the invention is described herein with reference to the
preferred embodiment, one skilled in the art will readily
appreciate that other applications may be substituted for those set
forth herein without departing from the spirit and scope of the
present invention. For example, a mirror may be placed on the
moving plate with a laser pointed at the mirror. This embodiment of
the invention is used to redirect the laser beam or to change the
link beam down in optoelectronic applications, or in a
microinterferometer. Another embodiment of the invention provides a
single plate that can be moved in both X- and Y-directions. This
embodiment of the invention employs a single spring, instead of a
folded spring, with one composite magnet placed underneath the
plate, and coils that drive the plate to move in both directions.
This embodiment of the invention may be used for such applications
as a microscope stage or for electronic applications where
micromirrors must be accurately positioned, i.e. this embodiment of
the invention provides a general purpose movable stage.
Accordingly, the invention should only be limited by the Claims
included below.
* * * * *